Chronic Diarrhoea
1. Definition
- Diarrhoea = ↑ stool frequency (>3/day) or ↓ stool consistency.
- Chronic diarrhoea = duration > 4 weeks.
2. Classification :
|
Type |
Mechanism |
Key Clue |
Improves with Fasting? |
|
Osmotic |
Unabsorbed solutes draw water |
High osmotic gap |
Yes |
|
Secretory |
Active chloride/water secretion |
Large volume watery |
No |
|
Inflammatory |
Mucosal damage |
Blood/mucus |
No |
|
Malabsorptive |
Impaired absorption |
Steatorrhoea |
Partially |
|
Motility-related |
Rapid transit |
Small frequent stools |
Variable |
|
Functional |
Dx of exclusion |
Pain Relived By stool |
No red flags |
Stool Osmotic Gap
It estimates the amount of unmeasured osmotically active particles in stool.
Normal stool osmolality ≈ 290 mOsm/kg (same as plasma).
We measure:
- Stool sodium (Na⁺)
- Stool potassium (K⁺)
Everything else is “unmeasured osmoles.”
Formula
Stool Osmotic Gap=290−2×(Stool Na+Stool K)
👉 Multiply by 2 because sodium and potassium are accompanied by anions (Cl⁻, HCO₃⁻).
|
Stool Osmotic Gap |
Type of Diarrhoea |
Mechanism |
|
>100 mOsm/kg |
Osmotic |
Unabsorbed solutes |
|
<50 mOsm/kg |
Secretory |
Active secretion |
|
50–100 |
Mixed |
Indeterminate |
Important Causes by System
|
Category |
Causes |
|
Inflammatory |
IBD, CRC, radiation |
|
Infectious |
TB, Giardia, C. difficile |
|
Malabsorption |
Coeliac, pancreatic insufficiency |
|
Endocrine |
Hyperthyroidism, Addison’s |
|
Drug-induced |
Metformin, SSRIs, PPIs, antibiotics |
|
Post-surgical |
Short bowel, bile acid diarrhoea |
|
Functional |
IBS |
3. Pathophysiology
A. Osmotic Diarrhoea
Mechanism: Non-absorbed solute retains water in lumen.
Causes:
- Lactose intolerance
- Sorbitol/fructose
- Magnesium-containing antacids
- Pancreatic insufficiency
- Coeliac disease
Key Point:
- Stool osmotic gap >100 mOsm/kg
- Stops with fasting
B. Secretory Diarrhoea
Mechanism: Active secretion via CFTR chloride channels.
Causes:
- Bile acid malabsorption
- Microscopic colitis
- Endocrine tumours:
- VIPoma
- Carcinoid
- Gastrinoma
- Chronic laxative use
- Post-cholecystectomy
Key Exam Features:
- Large-volume watery stool
- Continues during fasting
- Stool osmotic gap <50
C. Inflammatory Diarrhoea
Mucosal destruction → protein loss + blood.
Causes:
- Ulcerative colitis
- Crohn’s disease
- Colorectal cancer
- Chronic infection (TB, amoebiasis)
- Ischaemic colitis
Clues:
- Blood/mucus
- Tenesmus
- Weight loss
- Raised CRP
- Faecal calprotectin elevated
D. Malabsorption Diarrhoea
Fat and nutrients not absorbed → steatorrhoea.
Classic Causes:
- Coeliac disease
- Chronic pancreatitis
- Small bowel resection
- Giardiasis
- Small intestinal bacterial overgrowth (SIBO)
Clues:
- Bulky, pale, offensive stools
- Weight loss
- Vitamin deficiencies
- Iron deficiency anaemia
E. Motility Disorders
- Irritable bowel syndrome (IBS-D type)
- Hyperthyroidism
- Post-vagotomy
- Diabetic autonomic neuropathy
Functional diarrhoea
Functional diarrhoea is a chronic diarrhoea without structural, inflammatory, infectious, or biochemical abnormality.
It is classified under Disorders of Gut–Brain Interaction (DGBI).
Formal Definition (Rome IV Criteria)
Functional diarrhoea is diagnosed when:
- Loose or watery stools in >25% of stools
- Present for ≥3 months
- Symptom onset ≥6 months before diagnosis
- No predominant abdominal pain
- No evidence of organic disease
The key difference from IBS: pain is NOT a prominent feature.
Functional Diarrhoea vs IBS-D
|
Feature |
Functional Diarrhoea |
Irritable bowel syndrome (IBS-D) |
|
Abdominal pain |
Minimal / absent |
Present & recurrent |
|
Pain related to defecation |
No |
Yes |
|
Nocturnal diarrhoea |
Rare |
Rare |
|
Inflammatory markers |
Normal |
Normal |
|
Weight loss |
No |
No |
Red Flags In chronic diarrhoea patient
- Age >50 new onset
- Nocturnal diarrhoea
- Blood in stool
- Weight loss
- Iron deficiency anaemia
- Family history CRC
If YES → urgent colonoscopy + biopsy
Think:
- Ulcerative colitis
- Crohn’s disease
- Colorectal cancer
Differentiating Key Conditions
|
Feature |
IBS |
IBD |
Coeliac |
Pancreatic |
|
Blood |
No |
Yes |
No |
No |
|
Weight loss |
Rare |
Common |
Common |
Common |
|
Calprotectin |
Normal |
High |
Mild ↑ |
Normal |
|
Anaemia |
Rare |
Common |
Iron deficiency |
Possible |
|
Nocturnal |
Rare |
Yes |
Yes |
Yes |
|
Scenario |
Examination Findings |
Most Likely Diagnosis |
|
Chronic diarrhoea + iron deficiency anaemia,Bloating, weight loss, family autoimmune disease |
Pallor, dermatitis herpetiformis |
Coeliac disease |
|
Middle-aged woman + watery diarrhoea + normal colonoscopy,PPI/NSAID use |
Often normal exam |
Microscopic colitis |
|
Nocturnal watery diarrhoea |
May have weight loss |
Organic cause (IBD, secretory tumour) |
|
Bloody diarrhoea + tenesmus |
Tender abdomen |
Ulcerative colitis |
|
Skip lesions + perianal disease |
Perianal fissures, mass |
Crohn’s disease |
|
Post-cholecystectomy diarrhoea,Worse after meals |
Normal exam |
Bile acid diarrhoea |
|
Ileal resection history,Chronic watery stool |
Surgical scars |
Bile acid diarrhoea |
|
Steatorrhoea + alcohol history,Epigastric pain radiating to back |
Cachexia |
Chronic pancreatitis |
|
Diarrhoea + tremor + palpitations,Heat intolerance |
Goitre, tremor |
Hyperthyroidism |
|
Diarrhoea + hyperpigmentation + hypotension,Fatigue, salt craving |
Pigmented creases |
Addison’s disease |
|
Profuse watery diarrhoea (>3 L/day) + hypokalaemia |
Dehydration |
VIPoma |
|
Flushing + diarrhoea + wheeze |
Hepatomegaly |
Carcinoid syndrome |
|
Recent antibiotics |
Fever |
C. difficile |
|
Chronic diarrhoea + bloating + recent travel,Foul-smelling stool |
Mild tenderness |
Giardiasis |
|
Abdominal pain relieved by defecation + normal tests |
Normal exam |
Irritable bowel syndrome |
|
Elderly + weight loss + change in bowel habit |
Abdominal mass |
Colorectal cancer |
|
Diabetes + neuropathy + diarrhoea |
Peripheral neuropathy |
Diabetic autonomic neuropathy |
|
Radiation history pelvisRectal bleeding |
Skin radiation changes |
Radiation colitis |
|
Magnesium antacid abuse |
Normal exam |
Osmotic diarrhoea |
|
HIV patient + chronic diarrhoea |
Cachexia |
Opportunistic infection |